Just as the museum closes, Big Bird announces that he must stay behind to find Mr. Snuffleupagus. While the others search the entire museum for Big Bird, he and Snuffy befriend EgyptianPrince Sahu, who has been trying to solve a riddle for more than 4,000 years so that he can finally join his parents. The boy prince claims to be under a spell cast by Osiris, the Lord of the Underworld, and asks Big Bird and Snuffy to help him find the answer to the question, "Where does today meet yesterday?" The three wander throughout the museum searching for clues from ancient Egyptian drawings, while the others examine masterpieces and art objects.